National Technical University Of Athens Research Articles

Anchors to solid clay brick masonry in shear: Behavior under monotonic and cyclic loading

To investigate the behavior of injected anchors installed in solid clay brick masonry, an extensive experimental campaign was performed at the National Technical University of Athens (NTUA). The focus of the research is on the behavior in seismic conditions, to contribute to the design of relevant retrofitting applications. In this paper, the results of shear tests on metal injection anchors under monotonic or cyclic loading are presented. The anchors are installed in uncracked or cracked solid clay brick wallettes subjected to compressive stress of 0.2 MPa. The tests are performed under displacement-controlled conditions. In this part of the experimental campaign the main parameters investigated are the anchor location (in mortar joints or center of bricks), the presence of a crack crossing the anchor, the substrate properties (wallettes made with two different solid bricks, laid either in strong or in weak mortar) and the loading history (monotonic or cyclic). The experimental results illustrating the effect of the investigated parameters are presented, while the effect of cracking due to seismic loading and the reduction of shear resistance due to cycling are evaluated.

Cultural heritage structures and infrastructures vibration monitoring: vibration sensors metrological characteristics identification through finite elements modelling and simulation

The preservation of historical structures and infrastructures requires a multidisciplinary approach, based on preventive and planned conservation actions. For such a purpose, digital technologies are increasingly used to adaptively model and verify the behavior and stability of heritage assets. However, the reliability of model outputs depends on the accuracy on input data, which, in turn, depends on an appropriate choice of sensors, based on their technical characteristics. This is why an accurate design of monitoring system is crucial. Considering the installation and operation costs for a tailored system, it would be better to understand in advance the expected dynamic response of the structure or infrastructure to fit the monitoring system characteristics to the monitored structure . This is why, this work applies the use of FEM-based simulations to provide preliminary indications on the expected dynamic behavior of a heritage asset, supporting the identification of metrological parameters, in terms of sensors positioning and signals expected features. The used FEM, referred to a prototype of a Doric column, being the scaled 1:5 copy of a marble colonnade of the Temple of Apollo at Bassae (Greece), was designed, reproducing the parameterization (i.e., shape, measures, materials, etc.) of a previous FEM model, implemented and validated on the basis of experimental tests performed at the National Technical University of Athens. The results shed light on optimal sensor positioning and expected signal amplitudes, demonstrating the model's effectiveness in crafting tailored monitoring solutions for preventive conservation. The study underscores the importance of integrating Finite Element Modeling (FEM), avoiding the usual initial parallelization of model design and its experimental validation, into the creation of 'phygital' systems, that blend physical monitoring with digital twins. This approach not only enhances the accuracy of conservation efforts, but also suggests a promising direction for future research aimed at applying this methodology across a diverse range of heritage structures to support proactive preservation strategies.

Seismic design-assisted-by-testing approach for racks with dissipative baseplates in the cross-aisle direction

A design-assisted-by-testing strategy is proposed for the upright frames of adjustable pallet racks. This methodology defines a procedure that aims to locate marked post-elastic behavior at the floor-to-upright connections of lateral resisting frames in the cross-aisle direction. Such connections are intended to enhance the performance of the cross-aisle frames when subjected to dynamic forces in their plane. The chosen baseplate connection is experimentally tested using monotonic and cyclic protocols at the Steel Structures Laboratory of the National Technical University of Athens (Greece). Then, a numerical model of the baseplate is calibrated using literature parameter definitions and hence validated using physical data. Additionally, the dynamic response of two upright frames is numerically investigated: one with traditional hinge connections and another with dissipative ones. This study indicates that the proposed procedure successfully enhances ductility, resulting in a significant reduction (up to 50%) in upright axial forces compared to the conventional configuration. Finally, the open-source code for ground motion selection, together with its database, is released.

Influence of short lived radon and thoron progeny on the γ-spectroscopic analysis of low activity environmental and aerosol samples

In this work, the influence of radon and thoron progeny photons in γ-spectrometry analysis is thoroughly investigated. To this end three sampling and analysis campaigns were organized at the γ-spectrometry laboratory of Nuclear Engineering Laboratory of the National Technical University of Athens (NEL-NTUA), combining measurements of radon and its progeny via an ALPHAGUARD-ALPHAPM set up with background spectra of four germanium detectors as well as with outdoors meteorological conditions. Good statistical correlations were found between radon and its progeny with the respective photopeaks of the germanium detectors’ background as well as with meteorological conditions outdoors. Additionally, in all four detectors, six consecutive 2-day background spectra were compared with each other, with their mean and with the summed 12-day background spectrum, in order to study the photopeak background fluctuation and estimate the relevant uncertainties. This study supported the decision for the most suitable background spectrum to be used in γ-spectrometry analysis. Finally, in order to investigate the effect of both radon and thoron progeny on the MDA of typical radionuclides likely to be detected in aerosol samples, a series of air-sampling and γ-spectroscopic analysis scenarios were tested.

The Rehabilitation of the Historical Bridge of Konitsa, Epirus, Greece: A Documentation-Based Methodology of Structural Analysis and Rehabilitation Strategy

The bridge of Konitsa over the Aoos river, in Epirus, Greece, was built in 1869. It is one of the most important stone arched bridges in the Balkans, listed by the Hellenic Ministry of Culture (1982). The bridge, damaged by the Ottoman Army in 1913, was restored the same year by French engineers using reinforced concrete. Structural deterioration, located mainly in the area damaged by the explosion and subsequently repaired, led to the study of the bridge by the National Technical University of Athens (NTUA), in the framework of a Contract among the Prefecture of Epirus, the Municipality of Konitsa (owner of the bridge), the Ministry of Culture and NTUA. The entire study includes the exhaustive documentation of the bridge, its numerical modelling and assessment at its current state, the selection of adequate interventions and the numerical investigation of the efficiency of the proposed interventions. During this process, one of the main issues was the treatment of the concrete intervention of 1913. For the choice and for the design of the restoration measures, a calculation methodology was adopted, based on the findings of the documentation of the bridge, while taking into account the critical phases (construction, damage and repair) over its lifetime. This work has proven the available safety of the bridge under its self-weight, as well as the need for reconstruction of the RC jacket at the intrados of the arch, which was applied as a repair measure to the bridge in 1913.

An open source platform addressing structural stability risk assessment in historical centres

The management of stability risk assessment in historical centres in a project that aims to design and develop an open platform where blocks of buildings and large structures may be represented in damage maps before the event occurs, addressing damage forecasts for seismic movements impacting on the structural stability of the CH. The structural stability is calculated using a Matlab code developed at the National Technical University of Athens, translated to an open source programming language (Python)using the software tool GeoPandas to enable operations that would normally require spatial database software (PostGIS). Furthermore, the use of open source technologies like QGIS desktop software will provide open and free access to view and contribute in the future.

European Journal of Geography (Year 2023): Reviewer Appreciation & Publication Recap

European Journal of Geography (Year 2023): Reviewer Appreciation & Publication Recap

Mining, refining, and QSAR analysing the nanoinformatics in EPA NaKnowBase.

Concerns about the safety of manufacturing and using engineered nanomaterials (ENMs) have been increasing as the technology continues to expand. Efforts have been underway to investigate the potentially harmful effects of ENMs without carrying out the challenging empirical studies. To make such investigations possible, the US EPA Office of Research and Development (ORD) developed the nanomaterial database NaKnowBase (NKB) containing the detail of hundreds of assays conducted and published by ORD scientists experimentally investigating the environmental health and safety effects of ENMs (nanoEHS). This article describes specifics of the effort to mine, refine, and analyse the NKB. Here we use a quantitative structure activity relationship (QSAR) analysis, using a random forest of decision trees to predict the in vitro cell viability effects that occur upon exposure to ENMs that are similar in composition and structure and implement a set of laboratory conditions. These predictions are confirmed using the Jaqpot cloud platform developed by the National Technical University of Athens, Greece (NTUA) where nanoEHS effects are investigated with scientists working together globally.

UNI-axial dynamic tests on a stone masonry subassembly before and after interventions

The present study deals with the seismic response of three-leaf stone masonry subassemblies before and after the application of intervention techniques. In detail, shaking table tests are performed on a scaled substructure in the Laboratory for Earthquake Engineering, at the National Technical University of Athens (NTUA). The substructure consists of two parallel walls. One wall is of rectangular section, whereas the other is provided with a portion of transverse wall at its mid-length. A flexible timber floor rests on the top of the walls. Sine-sweep and dynamic tests are performed perpendicular to the two walls, before and after interventions. The aim of this work is to experimentally study the response of three leaf masonry walls against in-plane and out-of-plane seismic actions, to investigate the effect of the transverse wall to the out-of-plane behavior of walls (and vice versa) and to assess the effect of a selected set of interventions on the behavior of the substructure. The results of this study confirm the vulnerability of three leaf masonry structures, evidenced by the premature detachment of masonry leaves. They prove the significant role of the transverse wall, in reducing the width of the separation cracks observed in the walls subjected to out-of-plane actions followed by an increase of its in-plane damage, as well as the positive effect (in terms of load and deformation capacity) of grouting the walls and enhancing the diaphragm action of the floor. Additionally, the experimental results of this study can serve as a validation tool for the calibration of numerical models and as input for further analytical research. In this paper, the calibration of a smeared crack model that is based on macro-modeling approach is presented, along with a comparison between experimental and numerical results for the specimen before interventions, based on the performed calibration.

UAV-driven approach for assisting structural health monitoring of port infrastructure

Port engineering planning and management seek to confront challenges affecting the functional and structural capacity of port systems. To achieve this, design, maintenance, and rehabilitation strategies should be enhanced by dynamic approaches that help minimize the impacts of potential degradation due to climate change, natural hazards and aging of port infrastructure. Introducing Structural Health Monitoring (SHM) practices to asset management frameworks can improve port sustainability and resilience. Recently, new aspects for assisting SHM approaches with Unmanned Aerial Vehicle (UAV) applications have emerged. Therefore, the present research is focused on investigating the capabilities of an Unmanned Aerial Vehicle (UAV) monitoring program to advance current SHM practices for port infrastructure. For this purpose, four in-situ inspections were conducted by the Laboratory of Harbour Works (LHW) of the National Technical University of Athens (NTUA) at a Greek port in central-eastern Greece, namely Lavrio port. Data analysis was performed with Close Range Photogrammetry (CRP) methods and Geographic Information System (GIS) tools. The overall research showed that the UAV-driven SHM supports the condition assessment of port infrastructure, thus assisting practices for smart maintenance.

Anchors to Solid Clay Brick Masonry in Tension: Behavior under Monotonic and Repeated Loading for Constant Embedment Depth

This paper presents a part of an extensive experimental campaign performed at the National Technical University of Athens (NTUA) with the purpose of investigating the behavior of chemical anchors embedded in solid brick masonry. Anchors are tested in tension under monotonic or repeated loading. All tests are performed under displacement-controlled conditions. The experimental setup and the instrumentation are presented, along with the investigated parameters and the rationale for the selection of the values of those parameters. In this part of the experimental work, comprising fifty-six (56) tests, the examined parameters are the anchor locations (in mortar joints and in the center of bricks), the state of the substrate (cracked or uncracked), the width of the crack crossing the anchor or at its vicinity (up to 1.20 mm), as well as the loading history (monotonic or repeated). In the tests presented herein, the embedment length of the anchors is equal to 100 mm. The anchors are embedded in solid brick masonry wallettes, and subjected to a normal compressive stress equal to 0.20 MPa. The observed failure modes are explained, and the overall behavior of anchors subjected to tension is presented and commented upon, along with the effect of the investigated parameters on the measured tension resistance, and on the corresponding displacement.

Thermal efficiency enhancement of an Evacuated Tube Collector: Experimental, numerical and analytical approach

In this work a commercial Evacuated Tube Collector with U-pipe flow system and mini compound parabolic concentrators was investigated analytically, numerically and experimentally. The thermal function of the solar collector was tested experimentally in Athens city (Greece) at 26th of April for water inlet temperatures in the range of 50–75 °C. An analytical approach and a simulation model were utilized and validated through experiments. The comparisons showed mean deviations lower than 6 % from the experimental results, with the non-experimental solutions to appear a mean declination of 1.6 % between them. A detailed thermal analysis was followed using the validated analytical solution for an inlet temperature range of 10–80 °C. The volumetric flow rate was optimized analytically. Two different geometrical characteristics were modified for studying possible enhancement of the collector. These parameters were the number of the evacuated tubes and the diameter of each receiver tube considering the collector’s aperture area as constant. The increment of these parameters had positive effect on the thermal efficiency leading to enhancements greater than 6 %. The experiments were held at the Laboratory of Applied Thermodynamics (Mechanical Engineering Department) of National Technical University of Athens. The design and the numerical simulations of the collector were conducted with Solidworks.

The historical bridge of Plaka: Interpretation of structural behavior and collapse

The arched stone bridge of Plaka, built in 1866, is situated in the mountainous region of Epirus, Greece, and it bridges the river Arachtos. A significant part of the bridge, namely the central part of the arch and the east pier, have collapsed on February 1st, 2015, after heavy rainfalls in the region. The National Technical University of Athens (NTUA) has mobilized its staff and offered the documentation of the bridge, as well as the preliminary design for its restoration.The bridge was accurately modelled, in terms of geometry and properties of materials. Numerical calculations have revealed the cause of pre-existing damage, as well as the cause of collapse of the bridge.The thorough documentation and the assessment have proven that the bridge, at its original state, was able to safely withstand the actions expected to be exerted on it. Thus, the restoration was performed without alteration of the original structural system, and completed five years after the collapse.

Dynamic Modelling and Experimental Testing of a Dynamic Directional Amplification Mechanism for Vibration Mitigation

PurposeInertial amplification of an oscillating mass has been considered by various researchers as a means to introduce enhanced vibration control properties to a dynamic system. In this paper an experimental prototype of a novel inertial amplifier, namely the Dynamic Directional Amplification mechanism (DDA), is developed and its dynamic response is subsequently evaluated. The DDA is realized by imposing kinematic constraints to the degrees of freedom (DoFs) of a simple oscillator, hence inertia is increased by coupling the horizontal and vertical motion of the model.MethodsThe concept and mathematical framework of the amplifier are introduced and then validated with experimental measurements conducted on the vertical shaking table, located in the Dynamics & Acoustics Laboratory, National Technical University of Athens.ResultsAnalysis indicates the beneficial effect of the DDA to the dynamic response of the oscillator when compared to the initial structure, showcasing a decrease in the acceleration values and shift of the resonating frequency in the derived transfer functions.ConclusionsThe key novelty of the DDA lies in its inertial amplification properties, introduced by a simple geometry and easy-to-apply structure. The proposed framework may be incorporated in applications such as sound and vibration isolators, acoustic panels, acoustic and seismic metamaterials and other vibration control devices that aim to explore the DDA’s dynamic amplification properties. The mechanism has been previously applied by the authors to phononic and locally resonant metamaterials aiming to introduce bandgaps within the low-frequency domain.Graphical

Reversible protective and consolidating coatings for the ancient iron joints at the Acropolis monuments

This work presents a corpus of measurement methodologies utilized during a multidisciplinary project, aiming at the protection and conservation of ancient steel joints (clamps and dowels) of the Acropolis monuments, undertaken by the Acropolis Restoration Service (YSMA). The Laboratory of Physical Chemistry of the National Technical University of Athens collaborated with YSMA in designing and evaluating laboratory scale and field experiments, aiming in testing conservation materials. The first phase involved the application of seven coating systems on uncorroded metal coupons. The application characteristics, physicochemical properties (colour, gloss, hydrophobicity), and the protection performance against accelerated corrosion and polymer photo-oxidation were measured. Poligen CE12 and Paraloid B67 +2% nano-alumina were selected as the two most suitable materials for pilot applications on corroded steel joints and outdoor field exposure at the monument site. The assessment of the two coatings reversibility and of the corroded surfaces chemical alteration was undertaken by Raman spectroscopy analyses and orthophotographic documentation. During the 1-year field exposure, the corrosion development was retarded, but some local events of active corrosion could not be prevented. The joint areas coated with Paraloid B67 enhanced with 2% nano-alumina enabled more uniform application on corroded surfaces and exhibited better corrosion protection. In case of Poligen CE12, the nature of corrosion products indicated local acceleration of corrosion reactions.

In Depth Analysis of a Sediment Core from North Aegean Sea

In this work a thorough analysis of the vertical distribution of natural and artificial radionuclides and trace elements in a sediment core sample was conducted. A sediment core of 39 cm length was grabbed from the deep-sea trench (1540 m depth) southern of Samothrace Island, Aegean Sea. The sediment core was then treated at the Hellenic Centre for Marine Research, separated at 1cm increment samples, ranging from 0cm to 39 cm, dried, and milled. A first analysis of the samples was also conducted at HCMR using gamma spectroscopic techniques. Further spectroscopic analysis continued at Nuclear Engineering Department of the National Technical University of Athens (NED-NTUA) using an Extended Range Germanium Detector. The following radionuclides were determined 210Pb, 234Th, 226Ra, 228Ra, 228Th, 137Cs and 40K. Additionally, Instrumental Neutron Activation Analysis was used and the following major and trace elements were identified and quantified: Al, V, La, Sc, Mn, As, Ga, Cr, Zn, Co, Rb, Fe, K, Na and Ce. Based on the analysis of each sample, the vertical distribution of most natural radionuclides showed no significant variation. The vertical distribution of anthropogenic 137Cs was very low and almost constant. Unsupported 210Pb activity was about six times higher than 226Ra, indicating high radon fluxes. Furthermore, neutron activation analysis results indicate that almost all elements are within the expected range based on literature, while K and Na show a slightly increased concentration. For the area, these findings indicate steady and calm sedimentation processes.

Message from the Incoming Editor-in-Chief

While I officially started to assume the role of the Editor-in-Chief of IEEE Wireless Communications on July 1, 2022, Dr. Qian and I had been preparing for the transfer of the chief editorship for the past two months, and he had gracefully agreed to continue handling the special issues and submissions made while he was still the Editor-in-Chief. In my response to Dr. Qian's Farewell Message in the last issue (June 2022), I briefly laid out my plan for improving IEEE Wireless Communications. In this issue, I gladly report to you the progress on two fronts: the diversity of editors and the development of special topics. Five experts from diverse back-grounds have agreed to join the editorial board: Doru Calin (MediaTek US Inc., USA), Swades De (Indian Institute of Technology Delhi, India), Ai-Chun Pang (National Taiwan University, Taiwan), Dinh-Thuy Phan-Huy (Orange Innovation, France), and Symeon Papavassiliou (National Technical University of Athens, Greece); one from America, two from Europe and two from Asia; two from industry and three from academia; two females and three males. They are all distinguished individuals in our technical community. I have also formed a small team of experts, viz. Special Topics Development Editors, to proactively identify credible guest editors in organizing special issues on cutting edge topics: Doru Calin, Swades De, Dusit Niyato (Nanyang Technological University, Singapore), Ai-Chun Pang, and Symeon Papavassilou. Prof. Niyato joined the editorial board in 2021 and is one of the most responsive members of the board. I entrust this team to work closely with me to attract the readership of our magazine by publishing quality guest-edited feature articles on emerging topics. Dr. Calin, an outstanding industrial scientist, also steps up to be the new column editor for Industrial Perspectives; he has brought with him very rich industrial experience to fulfill this role. I would also like to use this opportunity to express my appreciation to Dr. Chun-Yen Wang, our long-serving Column Editor for Industrial Perspectives, who was tirelessly providing our readers with many years of industrial insights on many topics. I am closing this message by presenting to you their brief bios, and I am confident that you would agree with me that these distinguished board members will help us advance IEEE Wireless Communications.

An Algebraic Specification/Schema for JSON

An Algebraic Specification/Schema for JSON by Konstantinos Barlas1,* , Petros Stefaneas2 1 Department of Informatics and Computer Engineering, University of West Attica, Athens, 12243, Greece2 School of Applied Mathematical and Physical Sciences, National Technical University of Athens, Athens, 15780, Greece * Author to whom correspondence should be addressed. Journal of Engineering Research and Sciences, Volume

Ship Bow Wings with Application to Trim and Resistance Control in Calm Water and in Waves

Flapping foils for augmenting thrust production have drawn attention as a means of assisting ship propulsion in waves due to their high efficiency rate compared to traditional screw propellers. However, they can also offer substantial resistance reduction when used as stabilizers. In this work, the aim is to investigate the feasibility of a symbiotic concept combining the ship’s propeller with a foil arranged at the ship’s bow at a fixed position operating as a trim-pitch stabilizer. The work presents results obtained from experiments conducted in the towing tank of the Laboratory of Ship and Marine Hydrodynamics of the National Technical University of Athens (LMSH NTUA), as well as results from an in-house CFD solver. The test cases focused on the resistance and the dynamic behavior of the wing–vessel configuration in calm water conditions and in head waves. All results were compared against the performance of a bare hull (without foil). The findings of this work are based both on numerical and experimental data and indicate that a bow wing in static mode can be used for trim-control of a vessel by altering the angle of attack leading to a possible drop in wave resistance both in calm water and in waves. In the latter case, utilizing the wing in head waves results in a significant reduction in the pitching and heaving responses of the vessel, which may lead to substantial enhancement of the propulsion performance.

A Data-Driven Model for Pedestrian Behavior Classification and Trajectory Prediction

Pedestrian modeling remains a formidable challenge in transportation science due to the complicated nature of pedestrian behavior and the irregular movement patterns. To this extent, accurate and reliable positioning technologies and techniques play a significant role in the pedestrian simulation studies. The objective of this research is to predict pedestrian movement in various perspectives utilizing historical trajectory data. The study features considered in this research are pedestrian class, speed and position. The ensemble of these features provides a thorough description of pedestrian movement prediction, whilst contributes to the context of pedestrian modeling and Intelligent Transportation Systems. More specifically, pedestrian movement is grouped into different classes considering gender, walking pace and distraction by employing random forest algorithms. Then, position and speed prediction is computed employing suitable data-driven methods, in particular, the locally weighted regression (LOESS method), taking into account the individual pedestrian.s profile. An LSTM-based (Long Short-Term Memory) model is also applied for comparison. The methodology is applied on pedestrian trajectory data that were collected in a controlled experiment undertaken at the Campus of the National Technical University of Athens (NTUA), Greece. Prediction of pedestrian.s movement is achieved, yielding satisfactory results.